standing waves 
charles freeman 

Certain frequencies of strobing light are known to produce “standing waves” in the viewer’s visual cortex. These waves (and their superpositions) are experienced concurrently with the strobing light as lines, circles, spirals, honeycombs, and checkerboards. The viewer is invited to explore their own “flicker-phosphene hallucinations” by turning a dial that controls the strobe frequency and corresponding geometry.